Leak detector for immersion heaters



0ct. 14, 1952 l. M. MGNAIR LEAK DETECTOR FOR IMMERSION HEATERS Filed Aug. ll, 1950 /NVE/vro@ M. M cNA R AIT rop/ver FIG? ` other suitable materials.

- liquids are often stored in ceramic tanks.

Patented Cet. 14, `1952 LEAK DETECTOR FOR IMMERSION HEATER/S Irving M. McNair, Emmaus, Pa., assigner to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application August 11, 1950, Serial No. 178,820

This invention relates to the heating of liquids with immersion type electric heaters and particularly to protective features for eliminating the shock hazard incident to the use of such heaters.

When conventional immersion heaters are used to heat liquids in metal tanks, the operators can be protected from shock hazard, due to insulation breakdown in the heaters, in a simple and effective manner merely by groundingthe tank or the metal sheaths of the heaters, or both. In some cases, however, the liquids to be heated are corrosive or may be subject to contamination by any metal in Contact with them so that the tanks and the sheaths of the heaters must be made of Some commercially available immersion heaters are sheathed by fused quartz tubes, for example, and corrosive In cases of this kind, when minute cracks develop in thev heater sheath, the liquid seeps into the body of the heater and into contact with the electric heating element thereby causing the Whole body of liquid to be raised to a potential which is a hazard to the operating personnel. Since the tank and heater sheath are composed of insulating material, neither of them can be eiectively grounded nor can the body of liquid be grounded without introducing electrodes which would contaminate it or which are objectionable for other reasons.

The object of this invention is a simple and effective protective means for systems of this latter type. f

According to the invention, each such heater -is provided with an electrode preferably extending to the bottom thereof and normally insulated from the heater element. Any defect developing in the sheath will permit a small quantity of the solution to enter the heater thereby lowering the insulation resistance between the heater element and this added electrode. Sensitive relay means associated with these electrodes is operated by leakage current from the heater element of the defective unit and causes the power to be cut off from the heater before the solution in the tank reaches a dangerous potential.

In the drawing, Fig. 1 shows the invention as applied to a plurality of heaters in a single tank and Fig. 2 is a detailed view of a single heater modified according to the invention. The immersion heaters 3 as shown in Fig. 2 each may comprise essentially a coiled heater element 4 spirally wound on a ceramic core 5 and having lead-in wires 6 and 1` for connection to a source of current. The added electrode 8 may bein 6 Claims. (Gl. 219-38) 2 the form of a single wire extending from a lead 9 downwardly through a longitudinal bore l0 'in the ceramic core to the bottom of the fused quartz tube Il which serves as the outer sheath of the unit.

1 In Fig. 1 the tank I2 of ceramic or other non-- metallic lmaterial contains a solution I3 to be heated and is provided with superstructure (not shown) for supporting the required number of heaters 3 in the solution. Power is supplied to the heater elements 4 through a circuit breaker I4 from any suitable source which, for example, may be the star-connected secondary winding I5 of a three phase power'transforrner having its neutral point I6 grounded at 2| as shown. The heater elements are preferably connected to bus bars or cables H, I3 and I9 to balance the load on the several phases of the power supply and a fourth conductor 2|] is provided for connection to each of the added electrodes 8 of the heaters through the lead wires 1. The conductor 20'is connected to ground at 22 through a current limiting device 23 and a sensitive relay 24. The device 23 may be an adjustable resistor or, when power is derived from an alternating current source as in the case shown, this device may be a coil of suitable inductance to protect the relay from excessive current when one (or more) of the heaters becomes defective.

In normal operation the leakage current between the elements 4 and the corresponding electrodes 8 is, of course, negligible, but if the tube II of any heater should become cracked any appreciable amount of moisture accumulating in the tube in either liquid or vapor form will re-4 duce the insulation resistance between the eled ment 4 and the electrode 8. With the elements connected as shown, it will be noted that the lower portions of the elements where the moisture collects are well above ground potential so that due to the lowered resistance there will be appreciable leakage current through thek relay 24 to ground. The resulting operation of the relay opens the circuit breaker I4 thereby disconnecting the power supply from the heaters and removing any hazard that would otherwise develop. Experience with this type of protective system shows that the relay will be operated by seepage through hairline cracks too fine to be noticed by casual inspection and that this occurs before the surrounding solution becomes charged to a dangerous potential.

While it is feasible, if desired, to protect each heater with a separate relay, it has been found that at least 24 heaters may have their protective electrodes connected in parallel to one relay without setting up normal leakage currents sufcient to operate a relay of the sensitivity required to detect an incipient fault.

The manner in which the operation of the relay opens the power circuit will depend on the other protective, regulating and operating features of the system in which this protective circuit is used. In the circuit shown by Way of illustration, the circuit breaker is held closed against the action of the spring 25 by a holding coil 26 energized from the power source over a circuit extending through the normally closed contacts 21 of the relay 24. Upon operation of the relay the holding coil is deenergized and the circuit breaker automatically opens. If desired, the operation of the relay may cut 01T the power in any of various other known ways such as, for example, by closing normally open contacts to energize a trip coil in the circuit `breaker or by opening normally closed contacts in series with the contacts of the stop button normally used to disconnect power from the heaters.

Other modications of the embodiment of the invention shown by way of illustration may be made therein without departing from the scope and spirit of the invention as defined in the appended claims.

What is claimed is:

l. In a heating system for liquids, the combination with a tank containing liquid to be heated, an electric heater having an insulating sheath enclosing a heater element immersed in the liquid and a source of power connected to the heater element, of an auxiliary electrode within the sheath normally insulated from the element and a circuit connected to the electrode and energized upon a leakage o the liquid through the sheath for disconnecting the heater element from the source of power.

2. In a heating system for liquids including a tank containing liquid to be heated, a heater having an electric heating element and a non-metallic enclosing sheath immersed in the liquid and a source of power connected to the element, an electrode within the sheath insulated from the element and extending substantially to the bottom of the sheath, sensitive relay means connected to the electrode and adapted to be energized by leakage current between the element and the electrode and a Circuit controlled by the relay means for disconnecting the element from the source of power.

3. In a heating system for liquids, the combination with a tank containing liquid to be heated, a plurality of electric heaters immersed in the liquid and each having a heater element and an enclosing insulating sheath and a source of power connected to the heater element, of an auxiliary electrode disposed within each sheath but spaced fromv the heater element, a circuit interconnecting the auxiliary electrodes and relay means in the circuit for disconnecting the source of power from the elements, said means being of such sensitivity that it is not responsive to the total normal leakage current between the elements and the electrodes but is responsive to abnormal leakage current produced in a heater by leakage of the liquid into the sheath.

4. In a heating system for liquids, the combination with a tank containing liquid to be heated, an electric heater having an insulating sheath enclosing a heater element immersed in the liquid and a grounded source of power connected -to the heater element, of an auxiliary electrode within the sheath normally insulated from the element, a grounding connection for the electrode and means associated with the grounding connection and responsive to a leakage current between the element and the electrode due to seepage of the liquid into the sheath for disconnecting the source of power from the element.

5. In a heating system for liquids, the combination with a tank containing liquid to be heated, an electric heater having an insulating sheath enclosing a heater element immersed in the liquid and a source of power connected to the heater element, of an auxiliary electrode within the sheath normally insulated from the element and circuit connections including a current limiting device and a sensitive relay between the electrode and the power source and means responsive to the operation of the relay for disconnecting the element from the power source.

6. In a heating system. for liquids, the combination with a tank containing liquid to be heated, an electric heater having an insulating sheath enclosing a heater element immersed in the liquid and a grounded source of power for the heater element, of connections between the source and the element including a connection. from the ungrounded side of the power source to the bottom end of the element, an auxiliary electrode within the sheath normally insulated from the element and circuit connections including a sensitive relay between the electrode and the power source and means responsive to the operation of the relay for disconnecting the element from the power source.

IRVING M. MCNAIR.

REFERENCES CITED The following references are of record in` the Iile of this patent:

UNITED STATES PATENTS Number Name Date `1,365,978 Gallager Jan. 18, 1921 1,459,307 Laise et al June 19, 1923 1,602,743 Benson Oct. 12, 1926 2,535,064 Harrison Dec. 26, 1950 FOREIGN PATENTS Number Country Date 569,062 Germany Feb. 2, 1933 

